Dissolution of carbonates by acid has been practiced in a number of industries for years. For instance, well completions, cleanout and stimulation operations in wells and subterranean carbonate formations have used organic acids because of their low corrosivity. Such acids may be useful in the removal of calcareous materials deposited as scales within the formation during production of fluids at a variety of interstices of the formation rock itself as the fluid moves towards a well, in the production conduit of the well or in the fluid-handling equipment at the surface.
Further, more significant control of highly branched flow channels (called “wormholes”), created during acid matrix stimulation or acid fracturing of carbonate reservoirs, such as limestone, chalk and dolomite, have been observed using weak organic acids rather than hydrochloric acid, HCl. Further, use of a weak organic acid, such as acetic acid, permits for lower, and optimal, injection rate for wormhole generation, as well as the generation of larger wormholes. The slower reaction rates of weak organic acids also allow for retardation and, therefore, deeper penetration into the reservoir by the acid.
Under high temperature and pressure conditions, utilization of acids and acid choices becomes very selective because of corrosion control issues. Acetic acid has been preferred because of its low corrosivity property. However, reactivity of acetic acid under such conditions decreases dramatically due to a reduction in its dissociation constant. As set forth in Table I, as compared to HCl, under high temperature conditions, the dissociation constant of acetic acid, as well as formic acid, is reduced:
TABLE IDissociation ConstantAcid Name25° C.75° C.125° C.Acetic1.754 × 10−51.398 × 10−57.541 × 10−6Formic1.772 × 10−41.399 × 10−47.041 × 10−5Hydrochloric10Reaction efficiency of acetic acid, reported in the literature, demonstrates a range in values from 90% at 25° C. (77° F.) to 40% at 121° C. (250° F.) for 0.34 to 2.94 molal solutions (2 to 15 wt %).
The reaction of acetic acid and calcium carbonate is believed to proceed as follows:H++CaCO3Ca2++HCO3−H2CO3+CaCO3Ca2++2HCO3−H2O+CaCO3Ca2++HCO3−+OH−HAcH++Ac−When the partial pressure of carbon dioxide (a reaction product of carbonate and acetic acid) is low and the pH is low, the first equation dominates and when the pH is high, the third equation dominates. The second equation dominates the dissolution of calcite when the partial pressure of carbon dioxide is greater than 0.1 atmospheres and the pH is greater than 5
Chatelain et al., “Thermodynamic Limitations in Organic Acid-Carbonate Systems,” paper 5647 presented at the SPE 50th Annual Technical Conference and Exhibition, Dallas, Sep. 28–Oct. 1, 1975, reported a method of determining the extent of organic acid reaction on carbonate rock by thermodynamic equilibrium. Increased temperature and acid strength were disclosed to decrease conversion or reaction of organic acids on calcium carbonate. From their work, which involved the circulation of an acid with known initial strength through carbonate rock until equilibrium was attained, an approximation of the fraction of acid converted could be determined from the equation:
                              1.6          ×                      10            4                    ⁢                      K            D                          =                                            c                              CaA                2                                      ⁢                          c                              CO                2                                                          c            HA                                              (        I        )            wherein KD is the acid dissociation constant and C represents the concentration of the designated component in molality (gmole/kg water). See further Williams, B. B., Gidley, J. L. and Schechter, R. R.: Acidizing Fundamentals, Monograph Volume 6, SPE. In the actual stimulation of carbonate reservoirs however, no circulation occurs, only the injection of an acid system followed by a soaking or reaction period, followed by recovery. During the soaking period, equilibrium likely is attained.
Apparent from Chatelain's data is the fact that as acid strength is reduced, reactivity is increased. FIG. 1 shows the data published by Chatelain as the percent of initial acid strength converted and, in particular, shows the 2 wt % acetic acid curve being 65% converted at 121° C. (250° F.) and the 10 and 15 wt % acetic acid curves being 39 to 42% converted, respectively. Based upon equation (I), a 29 wt. percent acetic acid solution would be expected to convert 52%, 38.5% and 32.6% at 25° C., 75° C. and 125° C., respectively.
Since the disclosure of Chatelain, research has been consistently undertaken to develop means to improve upon the dissolution of carbonates. In particular, methods of increasing the reactivity of organic acids for use in carbonate formations and wellbores is desired in order that an increase in the amount of dissolution of carbonates can be attained.